Antorbital fenestra

An antorbital fenestra is an opening in the skull, in front of the eye sockets. This skull formation first appeared in archosaurs during the Triassic Period. Living birds today possess antorbital fenestrae, but the feature has been lost in modern crocodilians. In some archosaur species, the opening has closed but its location is still marked by a depression, or fossa, on the surface of the skull. This is called the antorbital fossa.

In dinosaurs, the antorbital fenestra is present on each side of the skull between the eye sockets and the nostrils, and in many genera it is the largest opening in the skull. The presence of the antorbital fenestra is a synapomorphy that unites tetanurantheropod dinosaurs as a clade. Alternatively, the absence of this opening is a trait distinguishing the dinosaur Protoceratops from other ceratopsians.[1] This fenestra is also absent in the hadrosauridae.